Hi, DR, not particularly large, 25-30l,,although a larger bag would be okay. I can barely fit a week's worth of gear into the side cases, would be nice to have extra room.

Quick attach/removal imperative.

In the 30 liter class one of my favorites is the BMW small tail bag (small in a misnomer).

The bag is basically a 30 liter bag with a waterproof inner membrane (use the inner or not as you wish)

It is expandable to about 35 liters if needed (I have seldom needed to).

It looks pretty decent on a BMW bike (mainly made for the BMW GS but it comes with lots of strapping extras to fit about anything).

One nice feature is IF your rear luggage rack/plate has a small rear lip behind the support stands there is a pocket on the bottom of the bag to slide over that lip so you only need to secure the front with quick disconnect buckles then slips off & is easily carried as luggage.

Size is around 13" tall x 16" wide x 13" long. (when full it also becomes a passenger back rest)

Might be worth a look at your local BMW dealer (they usually offer this bag on sale about twice a year & when on sale not a bad bargain for it's usefulness)

If mounting to the rear rack on my RTW or GSA this small BMW bag can't be beat! Looks great and on my 2 bikes fits like a glove on the back racks and pops on or off easily. Comes with various size straps for mounting which is really nice.

For use in the pillion I have used a Cortech Sport Tail bag like this but in black since 2004 and like it so much I bought a 2nd as a back-up but never needed it as the original is still going strong!

Have that Dry Spec bag,won it at Top O the Rockies BMW Rally 2 yrs ago.Great bag that can hold a ton of stuff,the internal stiffener(formed plastic) really helps with keeping shape and allowing easy access for loading,unloading.Also the stiffener on mine is white,which really helps when digging for gear.It is definitely waterproof.

Maybe wait a couple days,might be some good Black Friday deals coming up.

"This might fit the bill, although the zippers might be problematic...at least, they have been on my Rukka suit over the years: "

Try soft candle wax rubbed on the zippers (not beeswax). It sure helps "difficult" zippers of all types. I've used a small votive candle with a very "oily" wax for a long time. It stays in place (on the zipper), doesn't smell or stain.